The present invention generally relates to air cycle environmental control systems (ECSs). More specifically, the invention relates to an improved two spool ECS and improved method of conditioning water vapor bearing compressed air by utilizing two stages of water condensation/separation with no heat of condensation being absorbed in the air path downstream of a first turbine outlet and upstream of a second turbine inlet in two spool subsytems.
ECSs are used to provide a supply of conditioned air to an enclosure, such as an aircraft cabin and cockpit. In the past, an air cycle ECS has typically operated on a flow of bleed air taken from an intermediate or a high pressure stage within a jet engine having multi-compression stages. The bleed air has usually been pre-cooled within a primary heat exchanger with heat being dumped to RAM air and then flowed to a compressor. After compression, the air has been routed through a series of heat exchangers and condensers.. Then, the air has typically been expanded by a turbine which is mechanically engaged to the compressor. Finally, the air can be sent to the cabin.
Past air cycle ECS designs have included 2, 3 and 4 wheel air cycle machines, with high pressure water separation cycles. The general distinction among the three designs relates to the number of so-called wheels that are mechanically engaged to one another. All three of the ECS designs typically utilize a reheater and a condenser heat exchanger to respectively pre-cool the bleed air and then condense the water vapor in it. After condensation, the condensed water is removed by a water extractor. The resulting dehumidified air flows to the reheater where the remaining water droplets are evaporated, leaving the residual moisture in the vapor phase. The dry air then flows to a turbine for expansion and consequent cooling. The expansion will typically cool the air to below freezing temperature and thus the vapor particles form ice nuclei and crystallize into snow, which are swept downstream. The expanded air from the turbine can then be used to cool and condense water in the condenser heat exchanger.
For the 2 and 3 wheel system, the expanded air which has been warmed in the condenser can then be directly supplied to a cabin. However, the differentiating feature between those two systems is that the 2 wheel typically has the turbine engaged to a compressor, while the 3 wheel has the turbine engaged to the compressor as well as a fan that pulls RAM air through the system. In the 4 wheel design, shown for example in U.S. Pat. No. 5,086,622, the expanded air which has been warmed in the condenser is then further expanded by another turbine for eventual supply to the cabin. That design has the two turbines engaged to the compressor and fan, i.e., 4 wheels. Also, the design in U.S. Pat. No. 5,086,622 does not flow the dehumidified air through a reheater prior to entering the first turbine. That presents a disadvantage since the residual condensed water droplets in the first turbine inlet stream impinge on the cold turbine blades and outlet walls and freeze out if the metal temperatures are much below freezing. Ice then quickly accumulates and must be rapidly melted to avoid clogging the cycle. Another disadvantage is having the condenser upstream of the second turbine, which leads to the need for a condenser of large volume and heavy weight.
In a fashion somewhat similar to U.S. Pat. Nos. 5,086,622, 5,461,882 discloses a 4 wheel system that flows compressed bleed air from a compressor, to a reheater, and then to a condenser. Thus, water is being extracted at high pressure, similar to U.S. Pat. No. 5,086,622. From the condenser, dehumidified air is expanded in a second turbine where heat of condensation is recovered, and then flowed into a cabin. Alternatively, the dehumidified air from the condenser flows back through the reheater, to a first turbine, and back to the condenser. Disadvantages, however, include the fact that the condenser is again located upstream of the second turbine, which leads to the need for a condenser of relatively large volume and heavy weight.
A common disadvantage to the 3 and 4 wheel air cycle machine systems is that they create an xe2x80x9coff-designxe2x80x9d limitation. In particular, the fan is forced to operate at the same speed as the compressor and turbine(s), even though the fan typically finds optimal performance at a speed lower than the compressor and turbine(s). Thus, there must be a compromise in design optimization, which has usually been balanced in favor of the compressor and turbine(s). The 2 wheel system shown in U.S. Pat. No. 4,198,830 partially ameliorates the xe2x80x9coff-designxe2x80x9d limitation by incorporating a 2 spool design. In other words, the fan is engaged to a turbine by one spool and another turbine is engaged to the compressor by another spool. The spools operate independently of one another by having bleed air separately routed to each spool. Accordingly, the spools can be said to be operating in xe2x80x9cparallelxe2x80x9d to one another. Thereby, the fan can operate at a speed independent of that of the compressor and its related turbine, which has often been about one-fourth the speed of the compressor/turbine.
Yet, having spools parallel to one another in the 2 wheel system creates energy inefficiencies. With the parallel design, the fan and its related turbine operate off the bleed air before it is compressed and conditioned. In contrast, the compressor and its related turbine(s) operate off the bleed air upon being compressed and conditioned. Thus, during auxiliary power unit operation, while a majority of the bleed air (perhaps about 87%) is subject to being conditioned, it is not all of the bleed air. The consequence is that, among other things, the cooling capacity is reduced. Also, if only a small portion of the bleed air (perhaps about 13%) is going to turn the fan, there is less fan power as compared to a situation where all of the bleed air is used. Less fan power translates into requiring larger RAM air heat exchangers. Another energy inefficiency in the prior 2 wheel system is that the heat of condensation and sensible cooling is lost to the supply air. That is due to the fact that the supply air typically comes directly from the condenser, with no downstream means of recovery. Furthermore, the past 2 wheel system has typically provided no means for utilizing the spool containing the fan as an alternative conditioning spool in the event of a failure by the other spool.
U.S. Pat. No. 5,887,445 also provides two spools, but the spools operate in series rather than in parallel. In a high pressure spool subsystem, compressed bleed air moves through a reheater, next a condenser, and then a water extractor. Accordingly, water is being condensed in the condenser and extracted at high pressure. From the water extractor, a dehumidified air moves back through the reheater, into a high pressure turbine, and back through the condenser. From the condenser, the air can flow into a low pressure spool system and, specifically, a low pressure turbine. Upon expansion of the air, the low pressure turbine can then direct the air to an enclosure to be cooled. While the invention provides advantages, factors such as lower space requirements and fault accommodation with the low pressure spool failed can still be improved upon.
As can be seen, there is a need for an improved two spool ECS and method of conditioning high pressure water vapor bearing air which effectively increases cooling capacity by decreasing the required size of heat exchangers. There is an additional need for such a system and method that increases efficiency by recovering the heat of condensation and sensible cooling that might otherwise be lost to the supply air, for example. A further need is a two spool ECS and method that provides flexibility in use, including the ability to still provide conditioned air when one of the two spools is nonoperational.
In one aspect of the present invention, a two spool environmental control system comprises a low pressure spool subsystem comprising a low pressure turbine and a condenser downstream of the low pressure turbine. A high pressure spool subsystem is in air flow communication with the low pressure spool subsystem and includes the condenser, a first water extractor in air communication with the condenser, a high pressure turbine downstream of the condenser, a second water extractor in air flow communication with the condenser, and a reheater downstream of the high pressure turbine.
In another aspect of the present invention, a method of conditioning water vapor bearing bleed air comprises using a high pressure spool subsystem including a condenser and a high pressure turbine; using a low pressure spool subsystem downstream of and in air flow communication with the high pressure spool subsystem, the air flow communication being in the absence of rotating engagement between the subsystems, and the low pressure subsystem including a low pressure turbine and the condenser; condensing substantially all of the water vapor in the condenser and high pressure turbine such that a condensed water vapor is produced; and extracting the condensed water vapor upstream and downstream of the high pressure turbine.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.